1. Field of the Invention
The present invention relates generally to an expandable cover for protecting an expandable portion and/or articulated portion of machines or so forth from foreign disturbances, such as dust, light beam, moisture and so forth, particularly to a bellows. More specifically, the invention relates to a dust-proof device for a skirt portion of a seam-type bellows of the type which surrounds the expandable portion and/or articulated portion of the machines to be protected except for one side, typically the bottom.
2. Description of the Related Art
Conventionally, one-side opened seam-type bellows, typically a bottom opened seam type bellows is produced by cutting a material sheet which is produced by coating a neoprene rubber on a cloth, into an essentially rectangular configuration with both side extensions, as shown in FIG. 1 to form a plurality of C-shaped sheet 1;, overlying each two C-shaped sheets and sewing respective inner edges 2 and 3, 3 by a yarn 4, which will be hereinafter referred to as "inside sewing"; overlying the sewn pairs of the C-shaped sheets and sewing outer edges 5 and 6, 6 by a yarn 7, which will be hereinafter referred to as "outside sewing". By alternative inside sewing and outside sewing, a plurality of the C-shaped sheets 1 are connected with adjacent sheets in zigzag fashion to form one bellows, as shown in FIG. 3. As can be seen from FIG. 3, tip end edges 8, 8 of the both side extensions are left unsewn, which will be hereinafter referred to as "unsewn portion". The unsewn portions 8, 8 permit expanding and contracting deformation of the bellows by opening and closing deformation of the unsewn portions.
In the conventional one-side opened bellows constructed as set forth above, the outer edges and skirt portions 6, 6 of respective C-shaped sheets 1, 1 . . . repeat motion to displace inwardly in unison about their upper ends when the bellows expands (see solid line in FIG. 2) and to return to the original shape (see broken line in FIG. 2) when the bellows contracts. A displacement angle of each side edge 6, is shown as .alpha. in FIG. 2. The inward displacement angle .alpha. is caused by special coupling structure of the one-side opened bellows with the unsewn portions 8, 8.
The special coupling structure of the unsewn portions 8, 8 of respective C-shaped sheets 1, 1 . . . is as follow. Namely, the skirt portions of both outer edges 6, 6 of each C-shaped sheet 1, 1 . . . are positioned adjacent the unsewn portions 8, 8. The tip end of these unsewn portions 8, 8 are held horizontal at no load condition on the bellows, in which the bellows is contracted at the maximum contraction magnitude. Also, the outer end of the skirt portion is the terminating end of the outside sewing to mate with the outer end of the adjacent sheet at one side. On the other hand, the inner end of the skirt portion is the terminating end of the inside sewing to mate with the inner end of the adjacent sheet at the other side. Therefore, as shown in FIG. 3, according to expansion of the bellows, each of the unsewn portions 8, 8 is subject to tension force in mutually opposite directions at the outer end coupling portion and the inner end coupling portion, to open in zigzag fashion. Such contraction in the widthwise direction of each unsewn portion associated with the opening in the zigzag fashion is distributed as the inward displacement angle .alpha. of the outer edges 6, 6 of the C-shaped sheets.
It should be noted that to the skirt portion of both inner edges 3, 3 of the C-shaped sheets, outward tension force generated associating with the opening of the outer end coupling portion is exerted. Therefore, the inward displacement angle at the inner edges 3, 3 becomes smaller than the inward displacement angle .alpha. of the outer edges 6, 6 in the extent corresponding to the outward tension force. Even though the displacement angle is reduced in the certain magnitude at the inner edge as set forth above, the inward displacement of the inner edges 3, 3 inherently causes narrowing of the internal space of the bellows to possibly cause interference with a member to be covered(not shown). As a solution for this, the bellows is normally formed to be greater than the member to be covered so that the inward displacement of the inner edges 3, 3 may be accommodated without causing interference between the bellows and the member to be covered.
As set forth above, in the conventional one-side opened seam-type bellows, the unsewn portions 8, 8 of the C-shaped sheets are subject tensions in mutually opposite directions at the outer end coupling portion and the inner end coupling portion to open in the zigzag fashion. At the same time, due to reduction in width at the unsewn E portion due to opening at the coupling portion, both outer edges 6, 6 of the C-shaped sheet are displaced inwardly. The inward displacement motion of the inner and outer edge portions may serve as fan action. Simultaneous occurrence of such fan action and opening of the unsewn portion in the zigzag fashion should cause drawing force to introduce dust, water vapor, oil vapor and so forth floating around the bellows into the interior space of the bellows.
On the other hand, as set forth above, although the displacement magnitude at the inner edge 3, 3 is reduced due to outward tension force, as shown in FIG. 2 inward displacement at the unsewn portions is inherently caused to increase possibility of causing interference with the member to be covered. Therefore, in order to avoid the possibility of causing interference between the bellows and the member to be covered, the interior space of the bellows is formed to be greater than that required for covering the member so as to accommodate the inward displacement of the unsewn portion.